Why the transmission of signals using sky waves restricted to frequencies greater than 30 MHz?

Transmission of signals from the ground waves are restricted to 1,500 kHz. Why so?

Why sky waves are not used in transmission of TV signals?

Name the electromagnetic waves that have frequencies greater than those of ultraviolet light but less than those of gamma rays.

Why can sound waves be diffracted more easily than light waves?

Why are I.E. of 5d - elements greater than 3d- elements ?

We cannot hear beats.when beat frequency is more than 10.why?

Deep sea divers have been using the compressed air containing N_2 in addition to O_2 for breathing. When the sea divers breathes in compressed air at a depth, more N_2 dissolves in the blood and other body fluids than would dissolved at the surface because the pressure at the depth is far greater than surface atmospherice p ressure. when the diver comes towards the surface, the pressure decreases and N_2 comes out of the body quickly forming bubbles in the stream which restrict blood flow and affect the transmission of nerve impuses. this results into a condition called 'the bends' which is dangerous and painful. To avoid this condition, professionals now use air diluted with helium. As a student of chemistry, can you analyse as to why helium is used?